This invention relates to the construction of electric circuits on multiple-layer printed circuit boards having high wiring densities with through holes and vias disposed on a grid and, more particularly, to a circuit board and method for construction of such a board to provide for the inclusion of additional vias between points of the grid to access any one of a set of many wires, formed as strip conductors, passing between neighboring grid points for establishing electrical connection between wires on different layers of the board.
Printed circuit boards are employed frequently in the construction of complex electrical circuits. The boards may be formed as laminations of individual layers having electrical conductors etched from metal sheets. The metal sheets are insulated from each other by layers of insulating material. Connection between conductors on different layers is made by vias. Through holes passing through all the layers of a board are provided for mounting electrical components, such as encapsulated chip circuits and connectors, to the board.
In accordance with current manufacturing practice, all vias and through holes are located at points of a grid. This permits standardization of component sizes, particularly the sizes and spacings of legs which support a circuit component on a circuit board. The alternation of grid locations among the through holes and the vias maximizes the number of interlayer connection sites while maintaining full flexibility in selection of sites on a board for mounting a component.
The spacings between grid points has been sufficient to allow the passage of two wires, formed as strip conductors, between a through hole and a neighboring via. Thereby, as two of the strip conductors pass side-by-side along a layer of the board for connection between components mounted on the board, both of the conductors pass adjacent to vias to enable connection with another layer if desired. The conductor on the right may be electrically connected, as by soldering, to a via on the right, and the conductor on the left may be electrically connected to a via on the left. The paths travelled by the conductors among grid points located on a cartesian coordinate system may be parallel to an axis of the coordinate system, inclined to an axis, or a combination of parallel and inclined paths.
The efficiencies of such configurations of conductor paths are discussed in Nishihara et al, U.S. Pat. No. 4,298,770, issued Nov. 3, 1981. The most efficient routing is shown to include paths of varying inclination. With advancements in the technology of building printed circuit boards, it is now becoming feasible to construct narrower conductors, and to fit more than two conductors, possibly several conductors, in the standardized grid spacing between a through hole and a via. A passage between a through hole and a via, which passage may contain a plurality of conductors, may be referred to as a channel. Increasing the channel capacity is clearly advantageous because it provides a designer of printed circuit boards with greater flexibility in laying out the conductors, in connecting circuit components, and in locating such components.
A problem arises in that, with high channel capacity such as five conductors per channel, present circuit boards are limited in the amount of connections that can be provided to the individual conductors of a channel. The outer conductors pass adjacent to vias, as noted above, so as to make connection therewith as well as to terminals of circuit components. However, the inner conductors are isolated from the vias, and cannot be connected to a via until an outer conductor is terminated at a circuit component or at a via. This greatly restricts the utility of a many-conductor channel. Even a modified grid with additional vias disposed at fixed central points of grid elements would not offer appreciable advantage because such additional fixed-position vias, while presenting the opportunity for coupling to an inner conductor of a channel, would tend to block the passage way, and thereby generally limit the number of conductors which can be located in a channel.